FIG. 2 is a sectional view of a vertically aligned liquid crystal display apparatus showing an example of a liquid crystal display apparatus having a simple matrix type electrode structure. The vertically aligned liquid crystal display apparatus 1 includes a liquid crystal panel 2, and displays an image by using the liquid crystal panel 2.
In the liquid crystal panel 2, a first transparent substrate 5a configured by a glass plate or the like in which first strip-like transparent electrodes 3a made of ITO (Indium Tin Oxide) or the like are formed on one substrate surface, a first alignment film 4a is formed on the first strip-like transparent electrodes 3a, and a rubbing process is performed on the first alignment film 4a, and a second transparent substrate 5b configured by a glass plate or the like in which second strip-like transparent electrodes 3b made of ITO or the like are formed on one substrate surface, a second alignment film 4b is formed on the second strip-like transparent electrodes 3b, and a rubbing process is performed on the second alignment film 4b are bonded together so that the first alignment film 4a and the second alignment film 4b are opposed to each other, while using, for example, polymer spheres 6 as a spacer material, and the periphery is sealed by a frame-like seal 7, whereby a panel body 2a is formed. In the case of the liquid crystal panel 2, the first transparent substrate 5a which is in the lower side is configured as a rear substrate 5a on the side of a backlight (light source), and the second transparent substrate 5b which is in the upper side is configured as a front substrate 5b on the side of a display screen, thereby configuring a transmission type one.
In a gap (cell gap) between the first transparent substrate 5a and the second transparent substrate 5b, furthermore, a liquid crystal is encapsulated by the vacuum injection method, the dripping method, or the like, so that a liquid crystal layer 8 is formed. In the case of the liquid crystal panel 2, a nematic liquid crystal (negative liquid crystal) having a negative dielectric constant anisotropy is used as the liquid crystal, thereby forming a vertically aligned one.
Moreover, a first polarizing plate 9a is applied to the substrate surface of the first transparent substrate 5a opposite to the side which is in contact with the liquid crystal layer 8, and a second polarizing plate 9b is applied to the substrate surface of the second transparent substrate 5b opposite to the side which is in contact with the liquid crystal layer 8, thereby completing the liquid crystal panel 2.
FIG. 3 is a view showing an electrode structure of simple matrix driving in the vertically aligned liquid crystal display apparatus. In the first and second strip-like transparent electrodes 3a and 3b, for example, the first strip-like transparent electrodes 3a which are in the lower side are formed in parallel in a plural number in a lateral direction (horizontal direction) to be formed as a whole as a lateral stripe pattern to be configured as scanning electrodes that are connected to a scan driving circuit 10, and the second strip-like transparent electrodes 3b which are in the upper side are formed in parallel in a plural number in a longitudinal direction (vertical direction) to be formed on the whole as a longitudinal stripe pattern to be configured as signal electrodes that are connected to a signal driving circuit 11. The first and second strip-like transparent electrodes 3a and 3b constitute the simple matrix type electrode structure. In the liquid crystal panel 2, each of intersections of the selected lateral and longitudinal first and second strip-like transparent electrodes 3a and 3b which are formed in parallel in the lateral and longitudinal directions, respectively is set as one pixel. In other words, in the liquid crystal panel 2, pixels are arranged in a grid-like manner, the first and second strip-like transparent electrodes 3a and 3a are placed laterally and longitudinally with respect to the pixels which are arranged in a grid-like manner, and a voltage is applied while selecting required ones of the lateral and longitudinal first and second strip-like transparent electrodes 3a and 3a. As a result, the liquid crystals of the liquid crystal layers 8 (liquid crystal cell) of pixels at intersections of the selected lateral and longitudinal first and second strip-like transparent electrodes 3a and 3b are driven. Namely, the liquid crystal panel 2 has a simple matrix type electrode structure, and performs simple matrix driving (duty driving). In the case where the number of display pixels is large, the driving is time-division multiplex driving.
FIG. 4 is a view showing the arrangement structure of the polarizing plates of the vertically aligned liquid crystal display apparatus. In the first and second polarizing plates 9a and 9b, the first polarizing plate 9a has an absorption axis in the direction indicated by the arrow 9a′, and the second polarizing plate 9b has an absorption axis in the direction indicated by the arrow 9b′. Namely, the first polarizing plate 9a and the second polarizing plate 9b are placed so that their absorption axes 9a′ and 9b′ are perpendicular to each other (crossed Nicols arrangement), and the display mode of the liquid crystal panel 2 is normally black.
FIG. 5 is a view illustrating driving of the vertically aligned liquid crystal display apparatus, (a) is a view of a non-driven state, and (b) is a view of a driven state. In the liquid crystal panel 2, in the non-driven state where a voltage is not applied between the first and second strip-like transparent electrodes 3a and 3b, as shown in FIG. 5(a), the molecular major axis directions of liquid crystal molecules 8a in the liquid crystal layer 8 (liquid crystal cell) sandwiched between the first and second strip-like transparent electrodes 3a and 3b are substantially entirely aligned in an optical axis which is perpendicular to the first and second transparent substrates 5a and 5b by the functions of the first and second alignment films 4a and 4b. In order to, for example, control the alignment direction, however, the alignment is performed while forming a small tilt (pre-tilt), and there is little change in the polarization state of light passing through here. Therefore, light (linearly polarized light) passing through the first polarizing plate 9a which is on the side of the backlight of the liquid crystal panel 2 is incident substantially as it is on the second polarizing plate 9b which is on the side of the display screen of the liquid crystal panel 2, and mostly blocked or absorbed thereby to form a dark display (black display).
By contrast, in the driven state where a voltage is applied between the first and second strip-like transparent electrodes 3a and 3b, as shown in FIG. 5(b), the liquid crystal molecules 8a in the liquid crystal layer 8 sandwiched between the first and second strip-like transparent electrodes 3a and 3b are aligned by their negative dielectric constant anisotropy in a direction substantially perpendicular to the electric field, i.e., substantially in parallel to the first and second transparent substrates 5a and 5b, and change the polarization state of light passing through here. Therefore, light passing through the first polarizing plate 9a which is on the side of the backlight of the liquid crystal panel 2 emits from the liquid crystal layer 8 while the polarization state is changed, and hence the polarization component of the transmission axis which is perpendicular to the absorption axis 9a′ of the second polarizing plate 9b that is on the side of the display screen of the liquid crystal panel 2 is increased to form a bright display (white display).
Here, a DC current is harmful to a liquid crystal. When a DC current is continued to be applied to a liquid crystal, the liquid crystal material is deteriorated. In the used nematic liquid crystal (negative liquid crystal) having a negative dielectric constant anisotropy, alignment disturbances occur even at a relatively low voltage which is lower than a predetermined voltage, and therefore a pixel is driven by, for example, a signal leakage current to an adjacent pixel (this is called crosstalk), and the contrast is impaired. In the driving of the vertically aligned liquid crystal display apparatus 1, therefore, the applied signal is inverted at predetermined period in order to eliminate a DC component. In order to prevent crosstalk from occurring, furthermore, a predetermined OFF voltage that is equal to or lower than a critical voltage (threshold) at which the liquid crystal is driven is applied also to a non-selected pixel.
As described above, the vertically aligned liquid crystal display apparatus 1 includes: the first and second transparent substrates 5a and 5b which are opposed to each other to be placed through a predetermined gap; the first strip-like transparent electrodes 3a which are placed in parallel on the surface of the first transparent substrate 5a that is opposed to the second transparent substrate 5b, and the second strip-like transparent electrodes 3b which are placed in parallel in the direction perpendicular to the first strip-like transparent electrodes 3a, on the surface of the second transparent substrate 5b that is opposed to the first transparent substrate 5a; the liquid crystal layer 8 which is placed between the opposed surfaces of the first and second transparent substrates 5a and 5b, and which is configured by the liquid crystal having a negative dielectric constant anisotropy, the alignment of the liquid crystal molecules 8a being substantially perpendicular to the first and second transparent substrates 5a and 5b, and in which, when a voltage which is equal to or higher than the predetermined threshold voltage is applied between the first and second strip-like transparent electrodes 3a and 3b, the alignment of the liquid crystal molecules 8a is substantially in parallel to the first and second transparent substrates 5a and 5b; and the first polarizing plate 9a which is placed on the surface of the first transparent substrate 5a that is opposite to the surface opposed to the second transparent substrate 5b, and which has the absorption axis 9a′ in the predetermined direction, and the second polarizing plate 9b which is placed on the surface of the second transparent substrate 5b that is opposite to the surface opposed to the first transparent substrate 5a, and which has the absorption axis 9b′ in the direction perpendicular to the absorption axis 9a′ of the first polarizing plate 9a, and performs simple matrix driving (duty driving).
FIG. 6 is a schematic sectional view of the vertically aligned liquid crystal display apparatus in which the structures of the alignment films, the spacer material, the seal, the liquid crystal layer, the polarizing plates, and the like are omitted, (a) is a view showing the shapes of the conventional first and second strip-like transparent electrodes, and (b) is a view showing an electric field which is produced by the OFF voltage that is applied between the conventional first and second transparent electrodes of (a). In the first and second strip-like transparent electrodes 3a and 3b which are used in the thus configured vertically aligned liquid crystal display apparatus 1, usually, as shown in FIG. 6(a), electrode end portions enclosed by the dotted lines are formed into a tapered shape with respect to the opposed surfaces of the first and second transparent substrates 5a and 5b, and the whole electrodes form trapezoidal shapes in which the opposed surfaces of the first and second strip-like transparent substrates 5a and 5b configure the bases, respectively.
In the first and second strip-like transparent electrodes 3a and 3b having a trapezoidal shape as described above, as shown by the solid arrows in FIG. 6(b), the electric field produced by the OFF voltage which is applied between the first and second strip-like transparent electrodes 3a and 3b is aligned substantially in parallel to the direction perpendicular to the first and second transparent substrates 5a and 5b in middle portions (pixel middle portion) of the first and second strip-like transparent electrodes 3a and 3b in a pixel, but, in end portions (pixel peripheral portion) of the first and second strip-like transparent electrodes 3a and 3b in a pixel, the electric field is not aligned substantially in parallel to the direction perpendicular to the first and second transparent substrates 5a and 5b, to be formed as an oblique electric field, and the alignment of the liquid crystal molecules 8a in a non-selected pixel is disturbed by the oblique electric field. As shown by the hollow arrows in FIG. 6(b), therefore, light leakage occurs in the periphery of a non-selected pixel, and the OFF transmittance is increased, whereby the contrast is lowered.
In order solve this, as disclosed in Patent Literature 1, for example, a technique is known in which a black mask is disposed on the surface of the second strip-like transparent substrate which is opposed to the first transparent substrate, and in a portion where the second transparent electrodes are not formed, the taper-shaped end portions of the second transparent electrodes are covered by the black mask, and light leakage in the periphery of a non-selected pixel is prevented from occurring, thereby improving the contrast.